In a typical RF communication system, a transmittal signal may travel from a transmitter to a receiver over multiple paths, for example a direct path and also a reflected path. Each path may be considered a separate channel which is subject to the effects of fading, dispersion, etc. Moreover, the combination of signals at the receiver can result in additional fading. Such operating environments are known as multipath fading environments. Direct sequence spread spectrum (DS-SS) receivers can operate in multipath fading environments. A DS-SS receiver typically includes a rake receiver, which demodulates a received signal using plural demodulation “fingers”, often referred to as rake fingers. Each rake finger demodulates the component signal from a number of the channel paths (such component signal referred to as a multipath component). The outputs of the rake fingers are combined for improved performance.
With multipath channels, a transmitted signal arrives in components, with each component having a different delay. The components can be distinguished and resolved if the delays are of sufficient duration. However, in order to demodulate the signals, the rake receiver must know the delay of each channel path.
Typically, a rake receiver operates in conjunction with a delay searcher and a delay tracker. The delay searcher analyzes a received signal and finds the delays. These delays are assigned to the rake fingers. However, in mobile telecommunications the channels may be subject to additional fading due to the motion of the receiver. A delay tracker tracks the delays assigned by the searcher between channel searches. Thus, while the searcher looks over a wide range of delays, the trackers look over a smaller range surrounding the assigned delays.
With such a configuration, problems may result during reassignment after a new search. One problem relates to loss of accuracy after the new search. Typically, a delay searcher uses less resolution than a delay tracker. When a new search is performed, the delay searcher may find a path with a delay that is close, but not exactly the same as that being tracked by the tracker. The path may actually be the same being tracked by the delay tracker. The difference in sensed delay is due to the lesser resolution of the delay searcher. Nevertheless, the delay searcher would assign the new delay to the rake finger, resulting in loss of accuracy until such time as the tracker subsequently adjusts the new delay.
Another problem relates to unnecessary relocation of rake finger delays. Typically, a delay searcher will assign a delay associated with the earliest arriving channel path to the first rake finger. Likewise, the next arriving channel path is assigned to the next rake finger, and so on. Due to fluctuation in relative signal strength of the changing channel paths, this can cause the same channel path to be reassigned to a different rake finger after subsequent searches. This reassignment takes time, and data may be lost during reassignment of the same channel path to a different rake finger.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.